The present invention relates to the field of air flow or velocity sensors and particularly to microbridge air flow sensors. Many air velocity sensors that are commercially available are commonly of the single hot wire or thermistor type and are typically mounted on the end of a long probe for insertion into the air stream. In the more recent prior art has been described a microbridge flow sensor comprising a pair of thin film heat sensors and a thin film heater. These semiconductor chip sensors are described in patents such as U.S. Pat. Nos. 4,472,239, 4,478,076, 4,478,077, 4,501,144, 4,548,078, 4,581,928 4,624,137, and 4,651,564 all assigned to the assignee of the present invention.
One example of this microbridge flow sensor prior art is shown herein in FIGS. 1, 2, 3, 4 and 5 taken from U.S. Pat. No. 4,501,144. The prior art invention shown comprises a pair of thin film heat sensors 22 and 24, a thin film heater 26 and a base 20 supporting the sensors and heater out of contact with the base. Sensors 22 and 24 are disposed on opposite sides of heater 26. Body 20 is a semiconductor, preferably silicon, chosen because of its adaptability to precision etching techniques and ease of electronic chip producibility. The embodiment includes two identical temperature sensing resistor grids 22 and 24 acting as the thin film heat sensors and a centrally located heater resistor grid 26 acting as the thin film heater. Sensors 22 and 24 and heater 26 are preferably fabricated of nickel-iron, herein sometimes referred to as permalloy, having a preferred composition of 80 percent nickel and 20 percent iron. The sensor and heater grids are encapsulated in a thin film of dielectric, typically comprising layers 28 and 29 and preferably silicon nitride, to form thin film members. In the embodiment shown in FIGS. 1 and 2, the sensor comprises two thin film members 32 and 34, member 32 comprising sensor 22 and member 34 comprising sensor 24, each member comprising one-half of heater 26 and having a preferred dimension of 150 microns wide and 400 microns long.
The embodiment of the prior art sensor further comprises an accurately defined air space 30 which results in air space effectively surrounding elements 22, 24 and 26. The effectively surrounding air space is achieved by fabricating the structure on silicon surface 36, thin film elements 22, 24 and 26 having a preferred thickness of approximately 0.08 to 0.12 micron with lines on the order of 5 microns wide and spaces between lines on the order of 5 microns, the elements encapsulated in a thin silicon nitride film preferably having a total thickness of approximately 0.8 microns or less, and by subsequently etching an accurately defined air space, preferably 125 microns deep, into silicon body 20 beneath members 32 and 34.
Members 32 and 34 connect to top surface 36 of semiconductor body 20 at one or more edges of depression or air space 30. As illustrated in FIG. 3, members 32 and 34 may be bridged across depression 30; alternately, for example, members 32 and 34 could be cantilevered over depression 30.
Silicon nitride is a highly effective solid thermal insulator. Because the connecting silicon nitride film within members 32 and 34 is exceedingly thin and a good insulator, it contributes very little to the loss of heat from heater 26, and nearly all the heat conducted from heater resistor 26 to sensing resistors 22 and 24 is conducted through air surrounding heater 26. Moreover, the supporting silicon nitride film has such a low thermal conductivity that sensing resistor grids 22 and 24 can be located immediately adjacent to heating resistor grid 26 and yet can allow most of the heat conducted to sensing resistor 22 and 24 from heater resistor 26 to pass through the surrounding air rather than through the supporting nitride film. Thus, sensing resistor grids 22 and 24 are in effect suspended rigidly in the air space near heater resistor 26 and act as thermal probes to measure the temperature of the air near and in the plane of heater resistor grid 26.
The operation of the prior art invention in sensing air flow can be described with reference to FIG. 2. Heater resistor grid 26 operates at a preferred constant average temperature difference of 200 degrees Centigrade elevated above the temperature of silicon chip 20. The power required by heater resistor 26 to achieve 200 degrees Centigrade above ambient temperature is very small, being less than 0.010 watt.
In the prior art embodiment at zero air flow velocity, thermal conduction from heater resistor grid 26, largely through the surrounding air space including air space 30, heats identical temperature sensing resistor grids 22 and 24 to an average temperature of about 140 degrees Centigrade or about 70 percent of the temperature elevation of heater element 26. In the embodiment illustrated, sensor grids 22 and 24 are precisely symmetrically located with respect to heater grid 26 so that at zero air flow they have identical temperatures and have no differences between their resistances.
With air flow present, upstream resistor sensor 22 will be cooled by the transportation of heat away from sensor 22 toward heater resistor grid 26, whereas downstream sensor 24 will be heated by transportation of heat toward the sensor from heat resistor grid 26. Consequently, a resistance difference between sensor resistances 22 and 24 will be present with a corresponding difference in voltage drop which is a measure of the air flow. Typical unamplified voltage differences can be as high as 0.1 volt at a 1500 feet per minute flow velocity.
Because of the exceedingly small thermal mass of the heater and sensor element structure and the thermal insulation provided by the thin silicon nitride connecting means to the supporting silicon body, and because of the surrounding air space, response time of the present sensor is very short, with response time constants of 0.005 second having been measured. Consequently, sensor elements 22 and 24 can respond very rapidly to air flow changes.
In the operation of the prior art sensor, heater 26 is operated at a constant temperature above ambient temperature, sensors 22 and 24 being operated at constant current, and the changing temperatures of sensors 22 and 24 are sensed as changes in resistance. Typical circuits for accomplishing these functions are illustrated in FIGS. 4 and 5.
The heater control circuit illustrated in FIGS. 4 uses a wheatstone bridge 46 to maintain heater 26 at a constant temperature rise above ambient as sensed by heat sunk reference resistor 38. As previously indicated, the constant temperature rise above ambient is preferably set at approximately 200 degrees Centigrade. Wheatstone bridge 46 is shown comprising heater resistor 26 and a resistor 40 in its first leg and a resistor 42, heat sunk resistor 38, and a resistor 44 in its second leg. An error integrator comprising amplifiers 48 and 50 keeps bridge 46 balanced by varying the potential across it and thus the power dissipated in heater resistors 26.
The circuitry of FIG. 5 monitors the resistance difference between downstream sensor 24 and upstream sensor 22. This circuitry includes a constant current source 52 comprising an amplifier 72 and a differential amplifier 54 comprising amplifiers 68 and 70. The constant current source drives a wheatstone bridge comprising two high impedance resistors 56 and 58 in one leg and the two sensing resistors 22 and 24 with a nulling potentiometer 60 in the other leg. The gain of differential amplifier 54 is adjusted by potentiometer 62. Output 64 provides an output voltage that is proportional to the resistance difference between the two sensing resistors 22 and 24.
One of the successful materials used in thin film layer fabrication for the sensors and heater is the nickel-iron material sometimes referred to as permalloy, having a preferred composition of 80% nickel and 20% iron. Permalloy thin film when laminated within a silicon nitride member is protected from oxidation by air and can be used as a heating element to temperatures in excess of 400 degrees Centigrade. Such a permalloy element has a thermal coefficient of resistance (TCR) of about 4000 parts per million at zero degrees Centigrade.
When extreme linearity of resistance versus temperature is a requirement it has been found that a problem occurs with the use of permalloy. FIG. 6 is a graphical representation of output versus mass flow and it illustrates the lack of accurate mass flow measurement that is obtained at different temperatures with a permalloy air flow sensor. In FIG. 6 the x-axis is mass flow in standard cubic centimeters per minute. At higher temperatures, the same mass flow gives a larger response except for low flow rates where the curves coincide. FIG. 7 is a graphical representation of a resistance versus temperature curve for permalloy. It can be seen from the curve that at two points on the curve where .DELTA.R.sub.1 =.DELTA.R.sub.2 then .DELTA.T.sub.1 &lt;.DELTA.T.sub.2 because of the nonlinearity of the TCR.
The nonlinearity, as illustrated in FIG. 7, affects the measured mass flow in two ways: (1) through the heater, and (2) through the detectors. With the heater, the circuit keeps the heater resistance difference constant (relative to an ambient sensing reference resistor) when the ambient temperature changes. Consequently, at higher temperatures, because the R vs. T curve has a steeper slope, the same resistance difference results in a smaller heater temperature differential with respect to the ambient, and this tends to reduce the voltage output response. With the detectors, however, the steeper R vs. T curve at higher ambients increases the detector resistance differences, and so increases the output signal for any given mass flow. These two effects tend to cancel at low flows, but the detector effect dominates at high flows, yielding the responses shown in the attached FIG. 6.
In the invention taught herein it is clear that a linear TCR structure is needed in which material can also be fabricated as thin film resistor and sensor. It has been discovered that by combining a small permalloy length of resistor film with a long length of platinum film in series, a very linear TCR characteristic can be obtained and an accurate mass flow measurement becomes possible. The use of a hybrid combination of permalloy and platinum yields a first order correction to bring the family of curves of FIG. 6 together at high flows as well as at low flows.